The present invention relates to a differential amplifier circuit having integrated bipolar transistors.
Various conventional differential amplifier circuits formed of bipolar transistors are known. FIG. 1 shows a differential amplifier circuit of this type. This differential amplifier circuit includes a differential pair of NPN transistors Q1 and Q2 whose bases are respectively connected to a pair of differential input terminals 41 and 42, and whose collectors are respectively connected to current output terminals 43 and 44. The emitters of transistors Q1 and Q2 are respectively connected to the collectors of NPN transistors Q3 and Q4, whose emitters are connected to constant current source 45 via resistors RE1 and RE2. The collector and base of transistor Q3 are respectively connected to the base and collector of transistor Q4. Resistors RE1 and RE2 each have resistance RE.
In the differential amplifier circuit with the above arrangement, when a voltage difference between differential input terminals 41 and 42 is 0, current I1 flowing through transistors Q1 and Q3 is equal to current I2 flowing through transistors Q2 and Q4, and the current value is 1/2 constant current IE. In a low input signal range, if voltage difference V.sub.in between differential input terminals 41 and 42 is increased, input current IB1 of input terminal 41 and base current IB4 of transistor Q4 are increased. As a result, collector current IC2 of transistor Q2 is increased, while collector current ICl of transistor Q1 is decreased, thereby obtaining input/output characteristics as shown in FIG. 2. Accordingly, input resistance R.sub.in of the differential amplifier circuit becomes negative, and its value is determined by resistors RE1 and RE2 of the emitter circuit formed of input transistors Q1 and Q2 and current amplification factor h.sub.FE thereof, as follows: EQU R.sub.in =-h.sub.FE .times.RE (1)
If h.sub.FE =100 and RE1=RE2=.degree..OMEGA., R.sub.in =-2 k.OMEGA..
As shown in FIG. 3, when signal source 61 having positive signal source resistance R.sub.s is connected to differential amplifier circuit 62 having negative input resistance R.sub.in, if a signal source voltage and an input voltage of differential amplifier 62 are represented by V.sub.s and V.sub.in, respectively, transfer characteristics can be represented by a resistance division of R.sub.S and R.sub.in, as follows: EQU V.sub.in /V.sub.s =R.sub.in /(R.sub.s +R.sub.in) (2)
If R.sub.s of equation (2) is constant, the relationship between R.sub.in and the transfer characteristics can be represented by a graph shown in FIG. 4. That is, when R=.vertline.R.sub.in .vertline., transfer characteristics provide an extremely large value, as follows: EQU V.sub.in /V=-.infin. (3)
wherein the transfer characteristics greatly change in correspondence with changes in R.sub.in, when RE=.vertline.R.sub.in .vertline.. In this case, input resistance R.sub.in (=-h.sub.FE .times.R) varies in correspondence with a variation in h.sub.FE of transistors Q1 and Q2 in the differential amplifier circuit, and hence the above transfer characteristics change greatly. Therefore, if R.sub.in of the differential amplifier circuit is -2 k.OMEGA., and the differential input terminals of the differential amplifier circuit are connected to a pair of output terminals of a signal source having a resistance substantially corresponding to R.sub.s =2 k.OMEGA., e.g., a Hall element of a Hall motor, signal source voltage V.sub.s cannot be transferred, as input voltage V.sub.in, with its amplitude kept substantially unchanged.